Neptunium is the first of the elements that is not generally considered to occur in nature (though minute quantities do exist in various ores of uranium and thorium as a result of complex nuclear reactions). I know of no applications for this element.

Sample from the Everest Set.
Up until the early 1990's a company in Russia sold a periodic table collection with element samples. At some point their American distributor sold off the remaining stock to a man who is now selling them on eBay. The samples (except gases) weigh about 0.25 grams each, and the whole set comes in a very nice wooden box with a printed periodic table in the lid.

Radioactive elements like this one are represented in this particular set by a non-radioactive dummy powder, which doesn't look anything like the real element. (In this case a sample of the pure element isn't really practical anyway.)

Poster sample.
This mineral, Aeschynite, appears in my Photographic Periodic Table Poster representing neptunium, because this highly unstable element can't reasonably be photographed. The rock probably contains on the order of a few atoms of neptunium at any one time, as part of the complex decay chain of the uranium that makes up a much larger fraction of the sample. In no meaningful way is neptunium itself visible in the sample, but sadly that's about the best you can do with an element like this. The sample photograph includes text exactly as it appears in the poster, which you are encouraged to buy a copy of.Source:SoCal (Nevada), IncContributor:Theodore GrayAcquired:20 September, 2005Text Updated:4 May, 2007Price: $29Size: 1.5"Purity: 0%

Smoke detector element.
The main listing for this sample is under americium, because that is the active ingredient in these small radioactive buttons used in ionization type smoke detectors. This sample had been in my collection for five years before reader Michael Okun pointed out an interesting fact: Americium-241 decays (with a half-life of 452 years) into Neptunium-237, which has a much longer half-life of 2.1 million years. That means neptunium is building up in my existing americium sample, and has been for years.

Michael calculates you get about a trillion new atoms of neptunium every year in a typical smoke detector, and very few of them decay. Much like fine wine, the sample gets better the longer you keep it! (Actually I hate wine, why not drink your grape juice before it goes sour, that's what I say. But anyway, it makes a good analogy, for those people who do think wine tastes better the older it gets.)

If someone has a really old smoke detector, like 20 or 30 years old, I'd love to get it. You know you're supposed to replace those things, they don't last forever, so I'd be doing you a service by taking it off your hands.

Unfortunately even 50 trillion atoms of neptunium still isn't much, only about 20 nanograms. On the other hand, the americium in the button only weighs about 260 nanograms to start with, so after only 50 years it would already be almost 10% neptunium, not too bad for such an exotic element. And when my collection has been gathering dust in a museum for about 500 years, this button will reach a milestone: It will be more than half neptunium. They'll have to move it from the americium shelf to the neptunium shelf and get a new, fresher smoke detector button to replace it. Gosh I hope someone remembers to do that, maybe I can get the Clock of the Long Now people to set an alarm or something.